This invention relates to an antenna having a tri-plate structure which utilizes a micro-strip antenna (MSA) as its emitting element.
FIG. 1A is a perspective view showing an example of a conventional planar antenna called ring micro-strip antenna of coaxial feeder type and FIG. 1B is a sectional view taken along the lines 1B--1B.
As shown in FIGS. 1A, 1B, in this planar antenna 100, a ring circular emitting element 101 made of micro-strip antenna element is formed on a side of a dielectric plate 102, a ground plate 103 made of metallic foil is formed on the other side of this dielectric plate 102 and then a core conductor 105 of a coaxial connector 104 is connected to a feeding point provided on part of the ring circular emitting element 101 such that an external conductor 106 of the coaxial connector 104 is connected to the ground plate 103. In the planar antenna 100 having the above structure, generally impedance matching between the ring circular emitting element 101 and the feeder line (core conductor 105 in the coaxial connector 104) is carried out by changing a ring ratio (b/a) between an outer diameter a and an internal diameter b of the ring circular emitting element 101.
However, if the ring ratio (b/a) is increased to ensure impedance matching between the ring circular emitting element 101 and the feeder line (core conductor 105 in the coaxial connector 104), a high impedance characteristic of more than several thousands is indicated, thus it is impossible to perform impedance matching by changing the ring ratio (b/a) widely. Although a variety of substitutive proposals have been presented, existing proposals contain such problems as a number of laminated layers is increased too much, a special matching circuit is required or the like. Additionally, it has not been easy to restrict cross-polarization component by ensuring symmetrical pattern by securing a symmetry in main mode.